In such an apparatus, a patient lies down on an examination bed, generally on the back. In antero-posterior presentation, an X-ray produced by an X-ray tube passes through the patient downwards and is received by a radiation detector situated beneath the bed. In lateral type examination, X-radiation penetrates the patient on one side and leaves via the other side. Antero-posterior type examination is used to measure the integrity of bones associated with the patient's pelvis: essentially the sacrum and the neck of the femur. Lateral type examination is used to measure the integrity of the spinal column. To keep down costs, installations including such apparatuses need to be capable of performing in both directions. Bone density measurements are used to evaluate bone porosity and thus the strength of bone under stress.
The technique of bone density measurement is known. It consists in irradiating the body of a patient with radiation at given energy; in measuring the attenuation of the radiation; and in repeating the examination with radiation at another given energy. Given the two phenomena whereby the energy of such irradiation is absorbed in the body: Compton absorption; and photoelectric absorption; it is possible to use such measurements to distinguish between different types of tissue depending on their absorption susceptibilities in each of these modes. Radiological absorption by tissue is determined in a space of two dimensions: a Compton effect dimension and a photoelectric effect dimension. Relatively speaking, it is known that bones absorb more in one of these modes while soft tissues absorb more in the other mode. Thus, by changing axis, one of these dimensions can be used to reveal bone density on its own while the other one reveals the amount of soft tissue in the body. To perform such examinations, it is necessary to have an X-ray tube available that emits at two different energies, and also to have a detector capable of distinguishing radiation attenuation at two different energies. In conventional radiology, two tests are performed successively in time, with the emission spectrum of the X-ray tube being changed from one test to the other, and with the bone density measurements being deduced therefrom. The results of these measurements are presented in images.
It is also known that this measurement technique can be combined with the use of gamma cameras. This leads to a simplification of the procedure in that only one test needs to be attempted. So long as a two-channel gamma camera is used, it is capable on its own of counting strikes at one given energy and at another given energy without confusing strike numbers. A gamma camera, also known as an "Anger" camera operates as follows. After passing through the patient, gamma rays are detected by means of scintillation detectors. The incident gamma radiation is transformed into light energy by exciting a scintillator constituted by a thallium-activated sodium iodide crystal (NaI(T1)). The scintillator crystal disposed in this manner on the path of gamma rays after they have passed through the body being examined, serves to transform the gamma rays into light rays. Downstream from the scintillator, the detector further includes a network of photomultiplier tubes. Each photomultiplier tube in the network serves to transform the light energy from the scintillator into electrical pulses. The amplitude of each electrical pulse is proportional to the incident light energy and thus to the energy delivered by the gamma photon to the crystal.
After amplification, electrical pulses are selected from a range of energies by means of an amplitude channel selector. Normally only one range or "amplitude window" is selected in a gamma camera. Nevertheless, the use of two windows is known, in particular with apparatuses specialized in measuring bone density by means of a gamma camera. In other words, a gamma photon passing through the body excites the scintillator crystal, is transformed into a light photon and thereby gives rise to an electrical signal via the network of detection photomultiplier tubes, with the amplitude of the electrical signal being a function of the incident gamma energy. It is thus possible to count such a scintillation as a strike at a specific one of the two energies at the location where it occurs. The location where the scintillation occurs is deduced by processing all of the electrical signals produced simultaneously at the moment of the scintillation by all or some of the photomultiplier tubes in the network, and by calculating the central of gravity thereof. Such processing is generally performed by a computer. After a certain length of time, it is possible to establish the fraction of strikes at one energy and the fraction at the other energy for a given region of the scintillator and consequently for a corresponding region in the body of the patient. Depending on whether the fraction is high or low at the location under consideration, it can be said that the gamma rays that have passed through the body at this location have passed through zones that are boney to a greater or lesser extent. By subsequent image processing, it is then possible to show up the corresponding bone quality., This type of examination is very useful, in particular for showing up troubles with osteoporosis.
To produce radiation at two energies simultaneously, it is possible, for example, to use an X-ray tube which emits a substantially continuous spectrum, e.g. over the range 20 keV to 80 keV, and to feed it with an anode voltage that is at substantially 80 kV relative to the cathode. It would also be possible to use two chemical sources. By then causing the emitted radiation to pass through a neodymium oxide filter (Nd.sub.2 O.sub.3) having a density of 0.4 grams per square centimeter (g/cm.sup.2) before it passes through the body of the patient to be examined, it is possible to obtain a two-photon spectrum having two energy peaks at about 35 keV and about 43 keV. Unfortunately, the filter has the effect of removing 97% of the X-ray photons emitted by the tube. Nevertheless, this is the price that must be paid to go from a continuous spectrum to a two-photon spectrum.
In full-body examination of a patient, it is known that X-radiation can be limited geometrically after filtering to a thin flat fan-shaped beam. During examination, the body of the patient is then displaced perpendicularly relative to the plane of the flat beam so that the beam scans the entire body. Under such circumstances, a network of photomultiplier tubes disposed in a row is placed facing the fan-shaped X-ray beam. The bases of the photomultiplier tubes in alignment with one another then constitute a detection segment. The larger the segment, the greater the number of tubes and the larger the field under examination. This detection segment is minutely adjusted in the plane of the fan-shaped beam at a predetermined distance from the two-photon X-ray source.
Unfortunately, for technological reasons, it is not possible to obtain a very large detection field. In practice, even with an embodiment having 24 photomultiplier tubes in alignment with one another, the resulting field is no more than about 20 cm. In other words, the portion of the body that is examined is the portion of a section of the body lying between the focus of the X-ray tube and the 20 cm detection segment. It will readily be understood that in order to make this section as large as possible, magnification effects should be reduced as much as possible, and so the detector should be placed as close as possible to the body of the patient.
This constraint then means that when the patient is lying on a bed during antero-posterior type examination, the detector, must be pressed against the underside of the bed. Whereas, for a lateral type examination, the detector must be placed as close as possible to the side of the patient being examined.
In an apparatus capable of performing both types of examination, the X-ray tube and the radiation detector are carried at respective ends of a C-shaped yoke.
To pass from one type of examination to the other, the moving C-shaped yoke is caused to slide. Problems of available space are then encountered during sliding. In one known system, provision is made when passing from antero-posterior examination to lateral examination to begin by moving the under-bed detector away from the tube, i.e. the detector is displaced relative to the yoke and in the plane of the yoke. Once this displacement has been performed, the equipment is moved from one incidence to the other by sliding the yoke. Then the detector is returned to its standard position relative to the tube. This technique suffers from the drawback that camera type detectors are very difficult to adjust. In particular, the location processing system for determining where scintillations occur on the detector as a function of the photomultipliers that have detected them is very sensitive to misadjustment or to incorrect resetting of the detector relative to the focus of the X-ray tube. Such a technique is therefore unreliable since images cannot be compared when going from one type of examination to the other on the same patient.
In addition, the two types of examination require the patient to be presented differently on the bed. For antero-posterior type examination it is necessary to displace the X-ray tube and the detector together horizontally and longitudinally along the patient, or transversely relative to the patient. In contrast, for lateral type examination, the dynamics of the system for changing examination type by sliding is such that the spinal column of the patient (i.e. the part to be examined) now lies well below the alignment of the X-ray tube focus and the detection segment, even if the detection segment is then vertical. One known way of solving this problem is to ask the patient to get off the examination bed so that a thicker mattress can be placed thereon for the purpose of raising the patient's spinal column vertically. In addition to the additional mattress handling, asking the patient to lie down and get up in this way gives rise to time-wasting activity, thereby making the apparatus relatively uneconomic. In a variant, changeover is not complete. Examination is then not completely lateral. The X-ray tube fires diagonally downwards towards the detector. It can be shown that under such circumstances, the examination is not very precise and that in addition because of the shape of the bed, the detector is then at a distance from the patient, thus reducing the effective size of the sector for examination purposes.
An object of the invention is to remedy the drawbacks of prior apparatuses while essentially maintaining the detector in a fixed position relative to the X-ray tube. The detector and the tube are then securely mounted and fixed to opposite ends of a yoke that is capable of sliding.